a) Field of the Invention
This invention relates to a reflecting film formed by applying high-reflectance silver as a reflecting layer on a transparent polymer film, and also to a reflector making use of the reflecting film.
b) Description of the Related Art
Reflecting film s and reflectors, which reflect incident light, are used in a variety of applications such as lamp houses for use as backlights in liquid crystal displays, reflecting mirrors for use in printers and facsimile systems, reflecting plates in lighting equipments such as fluorescent lamps, reflecting plates for use in combination with photographic strobes, and mirrors of makeup compacts. In fields where a small thickness and light weight are required or workability into a desired shape is needed, reflectors of the construction that a metal layer is arranged as a reflecting layer on a transparent polymer film and light is allowed to enter from a side of the transparent film are widely employed.
Silver has a high reflectance to light in both the visible range and the infrared range, and its electrical and thermal conductivities are the highest among metals. Silver therefore attracts attention as a visible-light reflecting material, heat reflecting material and electric wiring material. Although silver is generally free from oxidation in the atmosphere, it reacts with sulfur dioxide gas and sulfur in the atmosphere so that black silver sulfide is formed. Further, it also reacts with ozone to form black silver oxide (AgO).
As a method for preventing the conversion of silver into the sulfide form in the atmosphere, it is known to form silver into an alloy. For example, for electrical contacts, silver containing 3-40 wt. % of Cu, Cd-containing silver and silver containing 10 wt. % of Au are employed. For dental purpose, silver containing 25 wt. % of Pd and 10 wt. % of Cu is used. For ornamentation purposes, silver containing 5-20 wt. % of Cu is used. Performance of silver in actual use is well-known to those skilled in the art. Darely indicating a publication which contains a detailed description about the performance of silver in actual use, reference may be made, for example, to Yuzo Yamamoto (1982), "Kikinzoku No Jissai Chishiki (Practical Knowledge on noble Metals)", 72-153, The Toyo Keizai Shinposha Ltd., Tokyo, Japan.
As another method for preventing the conversion of silver into the sulfide form, it is also known, besides the above-mentioned alloying method, to cover silver with a metal layer, a metal oxide layer, a metal sulfide layer, an alloy layer, a primer resin layer, a protective resin layer or the like. For example, Japanese Patent Laid-Open No. 107547/1974 discloses a method in which subsequent to formation of silver as a film on glass, an alloy layer composed of Cu and Sn is applied, followed by application of a resin layer, whereby silver is prevented from corrosion and enhanced scratch resistance is imparted. The present inventors also disclosed in Japanese Patent Laid-Open No. 279201/1989 that a thin-film silver layer can be protected from deterioration by light, heat, gas and/or the like by arranging metal layers of aluminum, titanium or the like on both sides of the thin-film silver layer.
In recent years, high-reflectance reflectors making use of silver as reflecting layers have been increasingly employed in products led by lamp reflectors of backlight sources for liquid crystal displays and including reflectors of fluorescent lamps. These reflecting members are so-called reflecting plates (silver-based reflecting plates) having a layer construction of PET (polyethylene terephthalate)/thin-film silver layer/adhesive layer/aluminum plate or so-called reflecting sheets (silver-based reflecting sheets) having a layer construction of PET/thin-film silver layer/adhesive layer/thin-film aluminum layer/PET/light-shielding layer. In these reflectors, the covering of silver with PET, a transparent polymer film, and an adhesive layer has succeeded in protecting silver from its conversion into the sulfide form and oxidation upon exposure to the atmosphere, said conversion and oxidation having remained as problems for many years, and hence retaining a high reflection. For example, when some samples of the above-mentioned silver-based reflecting plates and silver-based reflecting sheets were left over for 1,000 hours in an constant-temperature chamber controlled at 80.degree. C., no discoloration into a black or yellow color due to the formation of the sulfide or the like was observed and their reflectances were not reduced. When some other samples of these silver-based reflecting plates and silverbased reflecting sheets were left over for 1,000 hours in an air-conditioned chamber controlled at 60.degree. C. and 85% RH (relative humidity), neither discoloration nor reduction in reflection was observed either.
In addition, the present inventors also conducted an ultraviolet (UV) ray exposure test on further samples of the above-mentioned silver-based reflecting plates and silver-based reflecting sheets under a "QUV Testing Equipment" (trade name) manufactured by Q-PANEL Corp., U.S.A. As a result, their reflecting surfaces were found to be discolored into a purplish red color. This color is apparently different from any conventionally-known color formed by the conversion of silver into the sulfide form or by oxidation, such as black, yellowish brown or yellow color, and is also different from a color produced through yellowing as a result of deterioration of the PET film itself under ultraviolet rays. The present inventors therefore decided to call a reduction in the reflectance of a thin-film silver, which takes place under exposure to light (ultraviolet rays), "photodeterioration". With a view to avoiding this photodeterioration, the present inventors disclosed in Japanese Patent Laid-Open No. 162227/1993 a reflector improved in the durability against light (ultraviolet rays), heat and the like without any substantial reduction in the reflectance to visible light. This reflector is obtained by applying a thin film of a silver-containing metal on one side of a flexible substrate having a transmittance of 10% or lower for light ranging from 300 nm to 380 nm in wavelength.
The present inventors proceeded with a further investigation about UV deterioration of reflectors each of which had a stacked structure of transparent polymer film/silver. As a result, it was surprisingly found that, even when exposed to visible light from which ultraviolet rays had been eliminated, the reflecting surfaces became also discolored into a purplish red color as in the case of exposure to ultraviolet rays. Moreover, the above photodeterioration by visible light was found to proceed very fast at high temperatures although it proceeded very slowly at room temperature. Accordingly, this deterioration will hereinafter be called "photothermal deterioration".
FIG. 1 is a transmission electron micrograph (cross-sectional TEM picture) of a cross-section of a sample which underwent photothermal deterioration. The sample is a reflecting film of the construction that a thin-film silver layer is arranged on PET, and was subjected to an accelerated deterioration test (accelerated photothermal deterioration test) for 300 hours at an exposure intensity of 500 mW/cm.sup.2 and a sample temperature of 100.degree. C. The thin-film silver layer is observed to have partially separated from the PET. Further, particles of several tens nanometers in diameter are observed at an interface between the PET and the thin-film silver layer, thereby indicating intrusion of these particles in the PET. As a result of an analysis of these particles by electron probe micro-analyzer (EPMA), they were found to be silver. Incidentally, a large void which is seen on a right-hand side in the picture is a void which is contained in a resin used for the fixing of the sample.
According to the above findings of the present inventors, characteristics of photothermal deterioration can be summarized as follows: (1) photothermal deterioration is a deteriorative phenomenon specific to an interface between a polymer film and a thin-film silver layer; (2) an analysis of a photothermally-deteriorated portion by EPMA does not detect any of sulfur, chlorine and oxygen which are all detected in conventional deterioration of silver; and (3) no deterioration is observed in the thin-film silver layer at any portion thereof other than the interface between the polymer film and the thin-film silver layer.